This invention relates to a process for manufacturing an article of a hypoeutectic aluminum-silicon alloy by shaping a semi-solid body of the alloy comprising spheroidal primary particles dispersed in a liquefied phase and, more particularly, to such semi-solid metal-forming process that comprises heating the alloy to produce the semi-solid body under conditions that inhibit formation of free silicon particles.
A typical microstructure of cast hypoeutectic aluminum-silicon alloy solidified under quiescent conditions comprises primary dendrites dispersed in a eutectic matrix. Alternately, it is known to stir the molten metal during solidification to produce a microstructure comprising degenerate dendrites or spheroids. A billet of the spheroidal microstructure may be advantageously reheated to a temperature intermediate the liquidus temperature and the solidus temperature to partially melt the alloy. At the intermediate temperature, the eutectic matrix is liquefied to form a semi-solid state composed of a mixture of spheroidal particles and liquid phase. The semi-solid metal may be readily formed by a forging-like thixocasting process wherein the semi-solid metal is pressed into a mold using relatively small force.
U.S. Pat. No. 4,106,956, issued to Bercovici in 1978, describes a process for an aluminum alloy that transforms a dendritic microstructure to a spheroidal microstructure without stirring. The process comprises heating the dendritic alloy at a temperature intermediate the solidus temperature and the liquidus temperature to liquefy the matrix and maintaining the partially liquefied alloy at the temperature for a time sufficient to transform the primary phase. U.S. Pat. No. 4,415,374, issued to Young et al in 1983, describes a similar process, but wherein the alloy is worked prior to heating to reduce the time at temperature required for transformation.
It has now been found that rapid heating of hypoeutectic aluminum-silicon alloy to a semi-solid condition produces free silicon particles. In casting processes that include completely melting the alloy, the formation of free silicon by rapid heating has not presented a problem because the free silicon readily dissolves to produce a homogeneous melt. However, dissolution of free silicon in a semi-solid alloy is retarded, so that free silicon particles may be retained in the product alloy. Referring to FIG. 1, there is shown a photomicrograph of a sample of an aluminum alloy that was rapidly induction heated to a semi-solid state and quenched. The alloy contained about 7 weight percent silicon and designated A357 by the Aluminum Association, Inc. The billet was initially cast with electromagnetic stirring during solidification to form a degenerate dendritic microstructure. Following rapid reheating, the microstructure comprises primary silicon particles. The presence of free silicon particles constitutes a hard phase that can reduce machineability and interfere with post-forming heat treatments. Also, it is a common practice to apply an anodization treatment to articles formed of A357 alloy to produce a protective oxide coating. The presence of silicon particles on the surface interferes with anodization and creates a defect in the resulting coating.
It is an object of this invention to provide an improved semi-solid metal-forming process for hypoeutectic aluminum-silicon alloy that inhibits formation of free silicon particles.
It is a more particular object of this invention to provide an improved semi-solid metal-forming process for producing an article of hypoeutectic aluminum-silicon alloy, which comprises heating a solid body of the alloy to produce a semi-solid state at a controlled rate effective to inhibit formation of free silicon particles.
It is an object of one aspect of this invention to provide a semi-solid metal-forming process for manufacturing an article of a hypoeutectic aluminum-silicon alloy, which process comprises heating a cast body having a dendritic microstructure at a rate that inhibits free silicon formation to a temperature effective to liquefy the matrix, while retaining a solid dispersed phase, and thereafter maintaining the alloy at the temperature for a time effective to transform the dendritic phase into a spheroidal phase suitable for thixocasting into a desired shape.